The present invention relates to downhole tools as used in the oil and gas industry and in particular, though not exclusively, to a downhole tool which includes a lock bar to allow movement of a sleeve within the tool over a predefined range.
While many downhole tools operate continuously through a well bore e.g. scrapers and brushes as disclosed in U.S. Pat. No. 6,227,291, it is more desirable to provide a tool which performs a function only when it has reached a preferred location within a well bore. An example of such a tool would be a circulation tool as disclosed in WO 02/061236. The tool provides a cleaning action on the walls of the casing or lining of the well bore. The cleaning action is only required after the casing has been brushed or scraped and thus the tool is designed to be selectively actuated in the well bore. Such tools provide the advantage of allowing an operator to mount a number of tools on a single work string and operate them individually on a single trip in to the well bore. This saves significant time in making the well operational.
Tools which are selectively actuable in a well bore commonly operate by having an element which can be moved relative to the tool when in the well bore. In the circulation tool of WO 02/061236, the element is a sleeve located in the cylindrical body of the tool. When run in the well, the sleeve is held in a first position by one or more shear screws. To actuate the tool, a drop ball is released from the surface of the well through the work string. On reaching the sleeve, the ball blocks the flow of fluid through the tool and consequently pressure builds up until the shear screws shear and the sleeve is forced downwards. The movement of the sleeve is then stopped when a lower ledge of the sleeve contacts a shoulder on the internal surface of the tool body.
Such tools have a number of disadvantages. The tools are generally limited to one actuable movement. If two sleeves are incorporated to overcome this, the shear screws of the second sleeve can operate prematurely under the shock created to shear the shear screws of the first sleeve. Additionally, it is difficult to machine a circumferential shoulder into the central bore of a tool. To overcome this, the body is generally provided in two parts with different bore diameters, so that when they are screwed together a shoulder is created. This two part construction is expensive and is prone to weakness at the point where the parts meet. The reduced bore diameter of the lower part also effects the flow rate achievable through the tool.
It is an object of the present invention to provide a downhole tool which obviates or mitigates at least some of the disadvantages of the prior art.
It is a further object of at least one embodiment of the present invention to provide a downhole tool in which movement of a sleeve is controlled within a well bore.
It is yet further object of at least one embodiment of the present invention to provide a downhole tool in which hydraulic damping occurs to prevent premature shearing within the tool following a shock.